1. Field of the Invention
The present invention relates in general to refrigerant transporting hoses, and more particularly to such hoses suitably used as pipings for coolers, air conditioners and the likes provided on automotive vehicles.
2. Discussion of the Prior Art
There is known a hose for transporting a refrigerant such as flon (fluorohydrocarbon and chlorofluorohydrocarbon). The hose has an integrally formed three-layer laminar construction consisting of an inner tube, an outer tube located radially outwardly of the inner tube, and a reinforcing fiber layer interposed between the inner and outer tubes. Generally, the inner tube is formed of acrylonitrile-butadiene rubber (NBR) or chlorosulphonated polyethylene (CSM), while the reinforcing fiber layer has a mesh structure formed by braided organic yarn such as polyester fiber, rayon fiber, or nylon fiber. On the other hand, the outer tube is formed of ethylene propylene diene rubber (EPDM) or chloroprene rubber (CR).
The known multi-layered hose, which is formed of rubber materials except for the reinforcing layer, has a high degree of flexibility. Because of this property of the rubber materials, the hose can be handled with ease, when used as rubber pipings, for example. In addition, the hose is free from deterioration due to harmful metallic substances such as metallic ions and/or metal chlorides originating from metallic pipings used in a refrigerating or cooling circuit.
However, rubber materials generally tend to have comparatively high gas permeability, that is, low resistance to gas permeation therethrough. Accordingly, the conventional rubber hose whose major component consists of rubber materials suffers from a problem that the refrigerant (e.g. flon) circulating through the hose leaks through its wall, so that the amount of the refrigerant in the circuit is progressively reduced. In other words, the refrigerating or cooling capability of the system employing the rubber hose is comparatively rapidly lowered in use. Accordingly, the system must be frequently charged with the refrigerant, to maintain the nominal rating or capacity of the cooling system. Recently, the destruction of the ozone layer of the earth atmosphere by flon causes an environmental problem. It is therefore desired that a refrigerant transporting hose exhibits improved resistance to refrigerant permeation therethrough. While the conventional refrigerant transporting hose exhibits excellent flexibility and resistance to harmful metallic substances, it is not satisfactory in terms of the resistance to the permeation of the refrigerant.
In an attempt to improve the resistance of the conventional rubber hose to the refrigerant permeation, it is proposed to use a resinous material such as polyamide resins for one layer of the inner tube of the hose, as disclosed in laid-open Publication Nos. 60-113885, 63-125885 and 63-152787 of unexamined Japanese Patent Applications, and laid-open Publication No. 61-200976 of Japanese Utility Model Application, for example.
Such polyamide resins have higher rigidity (higher modulus of elasticity) than rubber materials, and a hose whose inner tube includes a polyamide resin layer suffers from reduced flexibility as a whole. When that polyamide resin layer is formed with a thickness as small as possible to avoid considerable reduction in the flexibility of the hose, the resistance to the refrigerant permeation becomes insufficient. Thus, it is extremely difficult to obtain a refrigerant hose which exhibits satisfactory properties in terms of both flexibility and resistance to the refrigerant permeation.
When a resinous layer of the inner tube of the hose is formed of a polyamide resin such as nylon 6 or nylon 66, or copolymer of nylon 6 and nylon 66, the hose has improved resistance to the refrigerant permeation, but suffers from lower flexibility and resistance to harmful metallic substances than the conventional rubber hose. As the polyamide resin as indicated above is excellent in the resistance to refrigerant permeation, it is possible to reduce the thickness of the resinous layer for improving the flexibility of the layer to some extent while maintaining the required permeation resistance. Even in this case, however, the flexibility of the hose is not satisfactory as compared with the conventional rubber hose.
Furthermore, in the case where other polyamides such as nylon 11 and nylon 12 are used for forming a layer of the inner tube of a hose, the hose has a high degree of resistance to harmful metallic substances, but tends to have lower resistance to the refrigerant permeation than the hose having a layer formed of the above-indicated polyamide such as nylon 6 or nylon 66. Consequently, the thickness of the layer formed of nylon 11 or nylon 12 must be increased so as to provide the same degree of refrigerant permeation resistance. The resinous layer having the increased thickness accordingly reduces the flexibility of the hose, and disables the hose to be practically used.
As described above, none of the conventional refrigerant transporting hoses are practically satisfactory in quality, in particular, in view of the recent increasing demand for further improvement in the resistance to refrigerant permeation therethrough.